数控开关电源外文翻译

power supply cookbookMarty Brown1．The Role of the Power Supply within the System and Design ProgramThe power supply assumes a very unique role within a typical system. In many respects, it is the mother of the system. It gives the system life by providing consistent and repeatable power to its circuits. It defends the system against the harsh world outside the confines of the enclosure and protects its wards by not letting them do harm to themselves. If the supply experiences a failure within itself, it must fail gracefully and not allow the failure to reach the system.Alas, mothers are taken for granted, and their important functions are not appreciated. The power system is routinely left until late in the design program for two main reasons. First, nobody wants to touch it because everybody wants to design more exciting circuits and rarely do engineers have a background in power systems. Secondly, bench supplies provide all the necessary power during the system debugging stage and it is not until the product is at the integration stage that one says “Oops, we forgot to design the power supply!” All too frequently,the designer assigned to the power supply has very little experience in power supply design and has very little time to learn before the product is scheduled to enter production.This type of situation can lead to the “millstone effect” which in simple terms means “You designed it, you fix it ( forever).” No wonder no one wants to touch it and, when asked, disavows any knowledge of having ever designed a power supply.1.1 Getting Started. This Journey Starts with the First QuestionIn order to produce a good design, many questions must be asked prior to the beginning of the design process. The earlier they are asked the better off you are. These questions also avoid many problems later in the design program due to lack of communication and forethought. The basic questions to be asked include the following.From the marketing department1. From what power source must the system draw its power? There are different design approaches for each power system and one can also get information as to what adverse operating conditions are experienced for each.2. What safety and radio frequency interference and electromagnetic interference (RFI/EMI) regulations must the system meet to be able to be sold into the target market? This would affect not only the electrical design but also the physical design.3. What is the maintenance philosophy of the system? This dictates what sort of protection schemes and physical design would match the application.4. What are the environmental conditions in which the product must operate? These are temperature range, ambient RF levels, dust, dirt, shock, vibration, and any other physical considerations.5. What type of graceful degradation of product performance is desired when portions of the product fail? This would determine the type of power busing scheme and power sequencing that may be necessary within the system.From the designers of the other areas of the product1. What are the technologies of the integrated circuits that are being used within the design of the system? One cannot protect something, if one doesn’t know how it breaks.2. What are the “best guess” maximum and minimum limits o f the load current and are there any intermittent characteristics in its current demand such as those presented by motors, video monitors, pulsed loads, and so forth? Always add 50 percent more to what is told to you since these estimates always turn out to be low. Also what are the maximum excursions in supply voltage that the designer feels that the circuit can withstand. This dictates the design approaches of the cross-regulation of the outputs, and feedback compensation in order to provide the needs of the loads.3. Are there any circuits that are particularly noise-sensitive? These includeanalog-to-digital and digital-to-analog converters, video monitors, etc. This may dictate that the supply has additional filtering or may need to be synchronized to the sensitive circuit.4. Are there any special requirements of power sequencing that are necessary foreach respective circuit to operate reliably?5. How much physical space and what shape is allocated for the power supply within the enclosure? It is always too small, so start negotiating for your fair share.6. Are there any special interfaces required of the power supply? This would be any power-down interrupts, etc., that may be required by any of the product’s circuits.This inquisitiveness also sets the stage for the beginning of the design by defining the environment in which the power supply must operate. This then forms the basis of the design specification of the power supply.1.2 Power System OrganizationThe organization of the power system within the final product should complement the product philosophy. The goal of the power system is to distribute power effectively to each section of the entire product and to do it in a fashion that meets the needs of each subsection within the product. To accomplish this, one or more power system organization can be used within the product.For products that are composed of one functional “module” that is inseparable during the product’s life, such as a cellular telephone, CRT monitor, RF receiver, etc., an integrated power system is the traditional system organization. Here, the product has one main power supply which is completely self-contained and outputs directly to the product’s circuits. An integrated power system may actually have more than one power supply within it if one of the load circuits has power demand or sequencing requirements which cannot be accommodated by the main power supply without compromising its operation.For those products that have many diverse modules that can be reconfigured over the life of the product, such as PCB card cage systems and cellular telephone ground stations, etc., then the distributed power system is more appropriate. This type of system typically has one main “bulk” power supply that provides power to a bus which is distributed throughout the entire product. The power needs of any one module within the system are provided by smaller, board-level regulators. Here, voltage drops experienced across connectors and wiring within the system do not bother the circuits.The integrated power system is inherently more efficient (less losses). The distributed system has two or more power supplies in series, where the overall power system efficiency is the product of the efficiencies of the two power supplies. So, for example, two 80 percent efficient power supplies in series produces an overall system efficiency of 64 percent.The typical power system can usually end up being a combination of the two systems and can use switching and linear power supplies.The engineer’s motto to life is “Life is a tradeoff” and it comes into play here. It is impossible to design a power supply system that meets all the requirements that are initially set out by the other engineers and management and keep it within cost, space, and weight limits. The typical initial requirement of a power supply is to provide infinitely adaptable functions, deliver kilowatts within zero space, and cost no money. Obviously, some compromise is in order.1.3 Selecting the Appropriate Power Supply TechnologyOnce the power supply system organization has been established, the designer then needs to select the technology of each of the power supplies within the system. At the early stage of the design program, this process may be iterative between reorganizing the system and the choice of power supply technologies. The important issues that influence this stage of the design are:1. Cost.2. Weight and space.3. How much heat can be generated within the product.4. The input power source(s).5. The noise tolerance of the load circuits.6. Battery life (if the product is to be portable).7. The number of output voltages required and their particular characteristics.8. The time to market the product.The three major power supply technologies that can be considered within a power supply system are:1. Linear regulators.2. Pulsewidth modulated (PWM) switching power supplies.3. High efficiency resonant technology switching power supplies.Each of these technologies excels in one or more of the system considerations mentioned above and must be weighed against the other considerations to determine the optimum mixture of technologies that meet the needs of the final product. The power supply industry has chosen to utilize each of the technologies within certain areas of product applications as detailed in the following.LinearLinear regulators are used predominantly in ground-based equipments where the generation of heat and low efficiency are not of major concern and also where low cost and a short design period are desired. They are very popular as boardlevel regulators in distributed power systems where the distributed voltage is less than 40VDC. For off-line (plug into the wall) products, a power supply stage ahead of the linear regulator must be provided for safety in order to produce dielectric isolation from the ac power line. Linear regulators can only produce output voltages lower than their input voltages and each linear regulator can produce only one output voltage. Each linear regulator has an average efficiency of between 35 and 50 percent. The losses are dissipated as heat.PWM switching power suppliesPWM switching power supplies are much more efficient and flexible in their use than linear regulators. One commonly finds them used within portable products, aircraft and automotive products, small instruments, off-line applications, and generally those applications where high efficiency and multiple output voltages are required. Their weight is much less than that of linear regulators since they require less heatsinking for the same output ratings. They do, however, cost more to produce and require more engineering development time.High efficiency resonant technology switching power supplies This variation on the basic PWM switching power supply finds its place in applications where still lighter weight and smaller size are desired, and most importantly, where a reduced amount of radiated noise (interference) is desired. The common products where these power supplies are utilized are aircraft avionics,spacecraft electronics, and lightweight portable equipment and modules. The drawbacks are that this power supply technology requires the greatest amount of engineering design time and usually costs more than the other two technologies.The trends within the industry are away from linear regulators (except for board-level regulators) towards PWM switching power supplies. Resonant and quasi-resonant switching power supplies are emerging slowly as the technology matures and their designs are made easier. To help in the selection, Table 1–1 summarizes some of the trade-offs made during the selection process.1电源在系统中的作用和电源的设计过程电源在一个典型系统中担任着一个非常重要的角色。